1. Field of the Invention
The present invention relates to an image transfer medium carrier member for carrying a sheet or other type of member or medium to which an image to be recorded is transferred. The invention also is concerned with an image forming apparatus which incorporates such an image transfer medium carrier member.
2. Description of the Related Art
Nowadays, various types of image forming apparatuses are widely used, such as electrophotographic copying apparatuses, printers, and so forth. Such an image forming apparatus generally uses an image transfer medium such as a recording paper sheet or a plastic film to which a toner or ink is transferred to form an image to be recorded. During the recording, the image transfer medium is carried by a member which in this specification is referred to as an "image transfer medium carrier member".
The image transfer medium carrier member, when used in an apparatus such as an electrophotographic apparatus for example, is subjected to various mechanical and electric forces during the image forming and recording process including conveyance of the image transfer medium, charging for the image transfer, elimination of electrostatic charges, cleaning, and so on. The image transfer medium carrier member, therefore, is required to have durability and strength, both mechanically and electrically, as well as resistance to wear. Furthermore, the member also has to have excellent lubricating nature, as it is frictionally contacted by cleaning member.
In recent years, it has become a common practice to use, as a developing agent, a so-called fine toner having particle sizes not greater than 10 .mu.m, about 8 .mu.m in mean particle size. The use of such a fine toner requires a more strict conditions of cleaning, i.e., the work for removing such a fine toner from the image transfer medium, since it is not easy to remove such a fine toner.
Hitherto, various types of plastic films have been used as the materials of the image transfer medium carrier member, such as Teflon, polyester, polyvinylidene fluoride, triacetate, polycarbonate or the like.
It has been reported that conventional image transfer medium carrier member tends to be cracked by mechanical or electrical external force, or due to deposition of a machine oil. Any crack formed in the carrier member causes a local change in the electric characteristics, which undesirably allowed generation of defects such as transfer unevenness (non-uniform transfer of toner) or a local omission of transfer (generation of area where toner is not transferred at all).
As a measure for solving such problems, it has been reported to form, on the surface of the image transfer medium carrier member, a coating layer of, for example, a polyester resin. This solution, however, is still unsatisfactory, since voids or pin-holes tend to be generated in the surface coating layer to degrade the quality of the transferred image. In addition, the surface coating layer is worn down during repeated use, with the result that the clear image cannot be obtained.